1. Field of the Invention
The present invention relates to a hydraulic system for actuating at least two valves, particularly for actuating a conical-pulley transmission of a motor vehicle having a continuously variable transmission ratio (CVT) and with at least two valves controlled by means of total control pressure.
2. Description of the Related Art
Hydraulic systems for actuating at least two valves are already familiar. A control valve can be supplied with a constant pilot pressure, whereby the control valve changes the pilot pressure such that the total control pressure acts equally on both valves in order to actuate them. A continuously variable transmission can include an automatically varying transmission. Such a continuously variable automatic transmission includes a starting unit, for example a planetary transmission as a forward/reverse unit, a hydraulic pump, a variable speed drive, an intermediate shaft, and a differential unit. The variable speed drive includes two pairs of pulleys and a transmitting member. Each pair of conical pulleys includes a second conical pulley that can be displaced in the axial direction. The transmitting member, for example a steel thrust belt, a tensioning chain, or a belt, runs between the pairs of conical pulleys. Adjustment of the second pulley changes the running radius of the transmitting member and hence the ratio of the continuously variable automatic transmission.
Stepless automatic transmissions require a high-pressure level to be able to adjust the pulley of the variable speed drive at the desired operating speed and hence transmit the torque more or less freely with a sufficient base clamping pressure. The entire control can result by means of electrical activation that can feature proportional valves, for example actuated electrically, and a pilot pressure feed line that can simultaneously actuate a downstream valve, or if necessary actuate several downstream valves.
An object of the present invention is to provide an improved hydraulic system for actuating at least two valves, particularly to better actuate the two valves despite common control with only one control valve, and particularly to prevent overregulation of at least one of the valves.